IIS 8.5 Instant 404.0 error on MVC site

I have an MVC site which I have published to my server. It has two different roles for internal and external users. Externally (using www.oursite.com) the site works fine.

when we try to access it internally (http://SERVERNAME/application) it instantly throws a 404.0 error:

Detailed Error Information:
Module             IIS Web Core
Notification       MapRequestHandler
Handler            StaticFile
Error Code         0x80070002
Requested URL      http://localhost:80/OURMVCSITE
Physical Path      D:\OURMVCSITE
Logon Method       Anonymous
Logon User         Anonymous

Both sites point to the same directory (D:\OURMVCSITE) and run under the same app pool, with the same basic and advanced settings. I cannot see any differences other than one runs as an application under "Default Web Site" and the other runs under the "oursite.com" website.

The 404 is instant as well, even after I recycle the app pool, which makes the external url take a while to start again.

Where else can I look to diagnose / fix this?

I have enabled Failed Request Tracing for the Default Web Site, but the folder is empty. No failed request logs get created.

Thanks